ADVANCED TREATMENT / NITROGEN PHOSPHORUS REMOVAL

Reach strict discharge standards with advanced variants of classical aeration
ADVANCED VARIANTS OF CLASSICAL AERATION
In modern wastewater treatment plants, nitrogen and phosphorus removal is critical to achieve strict discharge standards. Below are advanced variants of classical aeration systems and Biological Nutrient Removal (BNR) processes.

A/O (Anaerobic–Oxic)

In the anaerobic zone, polyphosphate-accumulating microorganisms take up volatile fatty acids and release phosphorus; then in the oxic zone, phosphorus is retained in the biomass, providing Enhanced Biological Phosphorus Removal. Nitrogen removal is limited because there is no anoxic stage; therefore, in plants with Total Nitrogen targets, either transition to chemical phosphorus + Modified Ludzack–Ettinger process or addition of an anoxic stage is preferred.

A²/O (Anaerobic–Anoxic–Oxic / Phoredox)

The most common Biological Nutrient Removal configuration: Enhanced Biological Phosphorus Removal is initiated in anaerobic, nitrate returned from oxic zone is denitrified with carbon in anoxic, and nitrification is completed in oxic. Internal recirculation is typically in the order of 2-4Q; Return Activated Sludge is mostly fed to anaerobic and nitrate leakage is carefully managed. Often meets medium-low Total Nitrogen / Total Phosphorus limits; performance may fluctuate under carbon constraints.

MLE (Modified Ludzack–Ettinger)

Provides strong nitrogen removal with pre-anoxic + oxic arrangement: Nitrate coming from oxic to anoxic via internal recirculation is denitrified with raw water carbon. Since there is no anaerobic stage, Biological Phosphorus removal is not expected; for Total Phosphorus, coagulant is mostly supported or a separate Anaerobic / Enhanced Biological Phosphorus Removal line is added. Preferred for stable Total Nitrogen performance and operational simplicity even under low carbon conditions.

STEP-FEED MLE

An adaptation of MLE to flow and load fluctuations; raw water is distributed to different stages by step feeding. This maintains F/M balance in the oxic zone during peaks, increases contact of nitrate transported by internal recirculation with carbon, and reduces Total Nitrogen fluctuations. Effective in terms of energy/efficiency balance against rain-infiltration effects in large/urban plants.

BARDENPHO 4-STAGE

Deep denitrification is targeted with pre-anoxic → oxic → post-anoxic → re-aeration flow. In the post-anoxic stage, nitrate is further reduced using endogenous carbon of biomass; re-aeration adjusts Dissolved Oxygen at the outlet and reduces floating sludge risk in settling. There is no Enhanced Biological Phosphorus Removal; chemical polishing is required if Total Nitrogen target exists. A reliable classic for Total Nitrogen ≤5 mg/L targets.

BARDENPHO 5-STAGE

EBPR is also activated by adding an anaerobic initial zone to the 4-stage; thus Total Nitrogen + Total Phosphorus are reduced together. Entry of Return Activated Sludge to anaerobic and anoxic-oxic internal recirculations are carefully adjusted; availability of carbon/Volatile Fatty Acids determines performance. A proven design for reaching low Total Nitrogen and low Total Phosphorus values.

UCT (University of Cape Town)

To protect Enhanced Biological Phosphorus Removal from nitrate leakage, Return Activated Sludge is first fed to the anoxic zone; after nitrate in Return Activated Sludge is consumed here, the mixture passes to anaerobic. Thus, true anaerobic conditions for polyphosphate-accumulating organisms are provided and Phosphorus removal remains stable. Internal recirculation is adjusted from oxic to anoxic; Biological Phosphorus Removal safety is higher than A²/O, especially effective when Return Activated Sludge nitrate is high.

MUCT (Modified UCT)

An enhanced version of UCT with multi-stage anoxic/recirculation lines and sometimes step-feed. The aim is to maintain Biological Phosphorus Removal stability even in seasonal variability by keeping the anaerobic volume cleaner (NOx-free). More compartmentation and control needs arise in design; in return, reliable phosphorus removal and good Total Nitrogen are obtained even at low COD/P ratios.

VIP (Virginia Initiative Plant)

A Biological Phosphorus Removal + denitrification structure similar to A²/O; the difference is that it is configured to minimize contamination of the anaerobic zone with nitrate through management of Return Activated Sludge and internal recirculations (Return Activated Sludge is usually to anoxic, neutralized flow from anoxic to anaerobic). When sufficient Volatile Fatty Acids are available, Total Phosphorus is very stable, and Total Nitrogen is at A²/O levels. NOx leakage is carefully monitored in operation.

JOHANNESBURG (JHB)

A variant that uses anoxic polishing and/or sludge fermentation logic in Return Activated Sludge/recirculation lines to support Biological Phosphorus Removal and reduce nitrate short-circuiting. Even in wastewater containing low Rapidly Biodegradable COD/Volatile Fatty Acids, it stabilizes Phosphorus removal by preventing nitrate from Return Activated Sludge from disrupting the anaerobic zone. A practical alternative in municipal plants where carbon balance is critical.

WESTBANK

A configuration that optimizes nitrogen removal with internal recirculation and feed splitting (step-feed), with high stability at peak loads. Contact of nitrate with fresh carbon is increased in anoxic stages; nitrification is ensured in oxic and outlet Total Nitrogen fluctuations are suppressed. Good energy-performance balance in lines with large flows and variable hydraulic profiles.

IFAS-BNR (Integrated Fixed-Film Activated Sludge)

A hybrid structure where biofilm and suspended growth work together by adding high surface area carriers to existing activated sludge aeration/anoxic compartments. Thus, nitrifiers are protected with long effective SRT on the film, nitrification capacity and load tolerance increase without changing pool volume; Total Nitrogen performance improves significantly in cold/shock load. Typical carrier fillings are in the order of ~10–25% in anoxic, ~20–40% in oxic; fine screen and carrier retention screen are mandatory. Denitrification is achieved with proper Internal Recirculation/Return Activated Sludge management; a separate anaerobic compartment is required if Biological Phosphorus Removal is desired.